I haven't ordered anything from FLP.com, but in 2019 I did order through Galeries Lafayette and it went smoothly. Here is the email if you'd like to reach out to them. The last contact I had was in 2019 with an SA named Kristiana to order a pochette for my DB. If they can't help they can probably check the Rue Cambon boutique. That is how they located the grey DB I bought when I was in Paris in 2018. The brown is very pretty, good luck!
[email protected]
Yes, I'm in the US too. After I bought the DB in Paris in fall 2018 I really wanted a matching pochette. FLP wasn't selling online yet so a few months later I just emailed the SA I dealt with at GL while I was there. They were happy to help and it was easy. It never hurts to ask.
Just wanted to update and give a big thank you again to @charlottawill for providing me the direct email address for the Galeries Lafayette store! They responded to my email the next day, connected me to one of their Sales Associates, and I was able to purchase the bag - it was just shipped today

Funnily enough, the SA said they just received more stock of the DB 27 in Walnut brown with Beige Leather recently and it's one of the most popular colors. Clearly there is a discrepancy between what's available online and what's available in the stores.